A popular thin-film photovoltaic technology is called CIGS, which refers to a photovoltaic device having a p-type semiconductor photon-absorber layer containing at least Copper, Indium, Gallium, and Selenium and capable of generating electron-hole pairs upon absorbing photons. In a typical CIGS photovoltaic cell, a Copper-Indium-Gallium-diSelenide (CIGS) layer operates with a heterojunction partner layer to generate a photocurrent when exposed to light. The photocurrent is produced when minority carriers are attracted from the CIGS layer to the heterojunction partner layer. Additional layers, such as a substrate, top and back contact layers, passivation layers, and metallization, may be present in the cell for structural rigidity, to collect the photocurrent, minimize reflections, and protect the cell. CIGS cells may also be layered with photovoltaic devices of other semiconductor materials into a multijunction, layered, structure.
CIGS semiconductor thin film can be created by a variety of processes, both in vacuo and ex vacuo in nature. Deposition methods such as sputtering, co-evaporation, and combinations of sputtering and evaporation performed in vacuo have produced CIGS photon absorber layers with high demonstrated performance, but traditional means for fabricating absorber layer are perceived as slow and prone to defects. Both sputtering and evaporation may involve a reactive process to create the CIGS alloy film having desired stoichiometry. Slow fabrication speed can lead to high fabrication cost. Defects in an absorber layer can allow recombination of electron-hole pairs thereby reducing cell efficiency and increasing panel area required for a given electrical output. Further, defects may short-circuit part or all of the photocurrent, impairing function of individual photovoltaic cells and modules made from such cells. Defects therefore reduce manufacturing yield and increase fabrication cost for cells and systems.
Some methods of creating a CIGS absorber layer deposit CIGS directly. Other methods deposit precursor sublayers, such as layers of copper, layers of indium and gallium, and layers of selenium, that are reacted in-situ to form CIGS. Delivery of either CIGS, or the precursor sublayers, can be performed by a single source, or by a plurality of sources. Existing processes typically require that the cell remain in a deposition zone for a lengthy time to deposit and form an absorber layer of the desired thickness.
Many defects in CIGS solar-absorber layers initiate at the surface of the underlying contact layer when the elements are initially disposed on the surface; these defects originate at the bottom of the CIGS absorber layer. Defects originating at the bottom of the layer may propagate through the entire layer. Growing CIGS films to the desired thickness without termination can allow these defects to propagate through the thickness of the film; defects extending through the thickness of the film are particularly prone to cause short-circuit defects because later deposited layers may contact layers underlying the CIGS layer.
Traditional in vacuo processing of semiconductor materials is batch-oriented. Substrates and source materials are placed in a chamber, air in the chamber is pumped out, deposition is performed, air is allowed back into the chamber after deposition is completed, and the substrates are moved to further processing stations or deposition sources in the chamber are replaced in preparation for following steps. In order to reduce cost of photovoltaic cells by increasing the area of cell produced with each pumping cycle of the chamber, there is much interest in roll-to-roll processing. In roll-to-roll processing, substrate of a feed roll is unrolled within the chamber, passed through at least one deposition and reaction zone, and wound onto a take-up roll after passing through the deposition and reaction zone. In roll-to-roll processing, there is economic advantage in maintaining high substrate transport speed through the deposition zone. High substrate speed through a deposition zone while reaching a desired film thickness requires either an extended deposition zone length or a rapid deposition rate of the film.
Increasing deposition rates of traditional in vacuo CIGS deposition processes typically requires larger size or larger quantity of sources, or both, but the basic sequencing of deposition is typically unchanged and propagation of defects through the entire thickness of the CIGS layer may be enhanced at high deposition rates. Defects propagating through the entire thickness of CIGS that cause the short-circuit defects are particularly critical to large-area CIGS modules formed by monolithic integration. Unlike modules made with discrete cells that are sorted to match performance prior to module integration, a monolithically integrated module is processed from a contiguous section of photovoltaic material, and any defect contained therein can severely affect the performance of that module.
Aluminum is in the same column of the periodic table as Gallium and Indium, Aluminum therefore has some similar chemical properties to Gallium and Indium and these three elements can be considered as forming a group; these three elements are classed as group IIIB in the periodic table. Similarly, Sulfur and Tellurium are in the same column as Selenium and have some similar chemical properties; these three elements can be considered as forming a group and are classed as group VIB in the periodic table. Silver and Gold are in the same column as Copper, have some similar chemical properties to Copper, and can also be considered as forming a group, these elements are classed as group IB in the periodic table. Group Ib-IIIb-VIb semiconductors as described herein typically have a chalcopyrite crystal structure having two parts VIb atoms for one part group Ib and one part group IIIb. While each element in these groups has some similar chemical properties to the other elements of the group, they also have significant physical, electronic, and chemical differences, which influence the physical, electronic and chemical compounds formed with them.
CIGS is classified, along with many other materials, as a IB-IIIB-VIB compound semiconductor material because of the periodic table groupings of its constituent elements.